A good cover letter should be three to four paragraphs in length and should explain why you are a good fit for the position for which you are hiring.  It should be confident, but not arrogant, and should reflect skills beyond what is recited on your resume.  A cover letter is a writing sample, so it will be scrutinized by employers. Prepare it sufficiently in advance so that you can schedule an appointment with a Career Services Counselor to review it.